1. Field of the Invention
The present invention relates to a circuit arrangement for an electronic d.c. telegraph transmitter which includes a transmitting keying circuit, a current regulating circuit which operates in accordance with the principle of a continuous regulator and which serves to maintain a constant current on the transmission line, and a telegraph battery, wherein the transmitting keying circuit connects the telegraph battery to the transmission line in the timing of the binary data signals.
2. Description of the Prior Art
In existing telegraph and data networks the signalling and data exchange takes place on the connection lines by means of d.c. telegraph transmitters of high transmitting voltage. The subscribers are connected to the exchanges or the repeaters via duplex four-wire double current lines (e.g. .+-.60 V, .+-.20 mA) or semi-duplex two-wire single current lines (e.g. 120 V, 40 mA). In certain cases the subscribers are connected via duplex four-wire single current lines (e.g. 120 V, 40 mA).
Since the subscribers are connected to the exchange via connection lines of various types and lengths, an extension line resistor must be manually set for each subscriber. Arrangements for an electronic d.c. telegraph transmitter are already known in which the current prevailing on the connection line, the so-called line current, is maintained at a constant value by means of automatic regulating devices. This automatic regulating device can either replace the conventional extension line resistor, or can be advantageously connected between the telegraph battery and the transmitting keying circuit which keys the connection line.
A known circuit which maintains the line current constant operates in accordance with the principle of the continuous regulator. Here, independently of the line length or the resistance of the transmission line, the so-called loop resistance, the current consumed remains constant. The power which is not emitted to the connection line is converted into heat in the regulating circuit. Because of the large extension line resistance, this is particularly great in the case of short line lengths and in the event of a short circuit. Therefore, when an electronic d.c. telegraph transmitter is constructed in a modern integrated space saving circuitry technique, additional measures are required to discharge the power loss which has been converted into heat. Therefore, it is necessary, on the one hand, to provide large surface cooling bodies and on the other hand substantial air cavities which results in a sizeable space requirement.
In order to reduce the power loss, in electronic d.c. telegraph transmitters, it is also known to maintain the line current constant by using regulating circuits which operate in accordance with the principle of the switching regulator. This represents a keyed current regulation wherein a measuring device measures the magnitude of the line current and, in dependence upon the deviation of the actual value from the theoretical value, controls the keying ratio of an additionally keying contact. The keying necessitates a smoothing of the output current. This known regulating circuit involves the disadvantage that a higher expense is incurred due to the additional keying arrangement and difficulties occur in the requisite smoothing of the output current. Therefore, a filter choke is additionally required.